This Technical Specification provides guidance on the use of heat flux meters in fire testing applications, including the description and working principles of common heat flux meters and methods for their selection and maintenance. The guidance can also be applied to measuring heat flux from radiant panels and other large heat sources used to simulate the heat flux from a fire. It is applicable for all common testing purposes when measuring heat flux from radiant sources. This Technical Specification also provides basic theory and working principles of heat flux meters and methods for selection, use and maintenance of heat flux meters. Although it is particularly aimed at the application of heat flux meters in fire tests and experimental works concerning fire research, it can also serve as a guide for other research applications, for example, research on boilers, combustion processes, etc. Instruments, which measure the transient temperature of a solid body of known mass and heat capacity to infer the heat flux (slug calorimeter type), are not covered by this Technical Specification.